1. Field of the Invention
The present invention relates generally to a control scheme. More particularly the present invention relates to a method and apparatus for limiting a time of operation in a critical speed zone of turbomachinery. This invention also relates to an antisurge scheme for a recycle compressor when a compressor-expander set trips.
2. Background Art
Most turbomachines, such as compressors, gas turbines, steam turbines, and expanders inherently exhibit at least one critical speed where the rotational speed of the turbomachine excites a natural frequency of the turbomachine. Extended operation at such critical speeds must necessarily be avoided.
When the critical speed or speeds reside below the normal operating region of the turbomachine, a startup procedure involving high angular accelerations through the critical speed or speeds is carried out, thus minimizing the time of operation in a neighborhood of the critical speed or speeds. This neighborhood around a critical angular or rotational speed is known as a “critical rotational speed zone,” and is thus defined for the purposes of this application, including the claims. Critical speed zones for a particular turbomachine are disclosed by the turbomachine manufacturer.
Critical speed zones residing within the normal operating speed range of a particular turbomachine are less common than those residing outside this normal operating speed range.
An improved cryogenic process for liquefying natural gas is disclosed in U.S. Pat. No. 6,308,531 by Roberts et al., and is hereby incorporated in its entirety by reference. The process is also described in a paper presented at the 2007 LNG 14 conference. The title of this paper is “Technical Challenges during the Engineering Phases of the Qatargas II Large LNG Trains” by Chavez et al., which is also hereby incorporated in its entirety by reference. The improved process includes a gas refrigeration cycle using nitrogen for a refrigerant. As is well known to those of ordinary skill in this art, a gas refrigeration cycle makes use of a compressor and an expander or turbine. The expander is used to drop the pressure of the gas, but also serves to extract energy from the refrigerant via shaft power. Shaft power derived from the expander is used to provide at least a portion of the required refrigerant compressor power. Gas refrigeration cycles are covered in many undergraduate thermodynamics textbooks such as Fundamentals of Engineering Thermodynamics 6th ed. by Moran and Shapiro, John Wiley & Sons, Inc., publishers, ISBN-13: 978-0471-78735-8 which is hereby incorporated in its entirety by reference.
The gas refrigeration cycle used for producing Liquid Natural Gas (LNG) in the Roberts et al. process is a regenerative cycle. That is, a heat exchanger is used to cool the high pressure stream upstream of the expander using the relatively cold low pressure stream downstream of the cooling load.
A departure from text-book gas refrigeration cycles in the Roberts et al. LNG application is the use of a first compressor, driven by the expander, and a second compressor driven by a separate driver. Because of the energy provided by the second compressor to the gas stream, the expander produces sufficient power to fully drive the first compressor.
The gas refrigeration cycle of the Roberts et al. LNG process is the coldest of a plurality of cascaded refrigeration cycles. Hence, the gas refrigeration cycle is used to subcool the liquid natural gas below its saturation temperature.
Typically, a plurality of gas refrigeration cycles, arranged in parallel, is used in the LNG process. The compressors in the compressor-expander sets may be operated using a load-sharing algorithm such as those disclosed in U.S. Pat. No. 5,743,715 to Staroselsky et al., which is hereby incorporated in its entirety by reference.
Turbocompressors generally experience unstable operation at low flow rates. The instability takes the form of either stall or surge, with surge being the most common for industrial compressors. In surge, the flow through the compressor suddenly reverses direction. This results in large thrust loads that can damage thrust bearings and cause vanes to contact the compressor shroud. Relatively hot gases from the discharge side of the compressor are drawn back into the compressor where more energy is added from the rotor, increasing the gas temperature even more. Repeated surge is to be avoided. Surge control algorithms are described in the Compressor Controls Series 5 Antisurge Control Application Manual . . . Publication UM5411 rev. 2.8.0 December 2007, herein incorporated in its entirety by reference.
A control system for the refrigeration processes in the Roberts et al. LNG process is needed. A challenging aspect for this control system is avoidance of critical speeds for the compressor-expander sets used in the gas refrigeration loop. These compressor-expander sets typically have a plurality of critical speed zones, some of which reside within the normal operating speed range of the compressor-expander sets. Extended operation in these critical speed zones must be avoided, but the gas refrigeration process must not be disrupted.
When a compressor-expander set trips or is shut down for any reason, including that of residing too long in a critical rotational speed zone, the second compressor, driven by a separate driver, may be pushed toward surge.
There is, therefore, a need for an improved control system for a compressor-expander set.